[8.18] (backport #17293) fix: close default apm tracer

[mergify]

Motivation/summary

the default tracer is leaking and its background goroutine spamming requests to this apm server (default endpoint) If TLS is enabled it causes 'http request sent to https endpoint'.

Close the default tracer since it's not used.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

• run apm-server with tls enabled
• enable self instrumentation
• check the message is not logged on startup

Related issues

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This is an automatic backport of pull request #17293 done by [Mergify](https://mergify.com).

[endorama]

I tested with 8.18.3 BC1 and I got this log line printed:

{"log.level":"error","@timestamp":"2025-06-23T11:19:38.136Z","log.logger":"beater.http","log.origin":{"function":"net/http.(*Server).logf","file.name":"http/server.go","file.line":3638},"message":"http: TLS handshake error from [::1]:36942: client sent an HTTP request to an HTTPS server","service.name":"apm-server","ecs.version":"1.6.0"}

The same message does not appear if auto instrumentation is set to off.